Many of the problems with common diagnostic techniques for ear pathologies were overcome by a device which measures acoustic reflectance, which is a quantity related to the complex acoustic impedance of the middle ear. Acoustic reflectance also is used to analyze the nasal cavity or to measure volume of the lungs. This technology also may be used for veterinary purposes.
Acoustic reflectometry for the ear involves transmitting sound waves (called incident waves) through the ear canal to the tympanic membrane. The incident waves are selected from a range of frequencies including the resonance frequency of the tympanic membrane; ideally the amplitudes of the incident waves also are the same for all frequencies, but this result often is not achievable. Some of the incident waves are reflected off the tympanic membrane and other components of the ear. The reflected waves may be detected by a microphone. In order to detect resonance, the microphone detects the vector sum of the incident and reflected waves over the range of frequencies. The envelope of this vector sum, herein called an acoustic reflectance curve, has a dip, also called a null. In the literature, this diagnostic technique is generally referred to as acoustic reflectometry or acoustic otoscopy and the device is generally referred to as an acoustic reflectometer or acoustic otoscope.
A suitable device and methodology for measuring acoustic reflectance of an ear are described in U.S. Pat. Nos. 4,601,295 and 4,459,966 to John H. Teele (the Teele patents). Such devices were made commercially available by ENT Medical Devices, Inc., of Wareham, Mass., and Endeco, Inc., of Marion, Mass. With these devices, the amplitude of a peak of the null of the acoustic reflectance curve, called the null value, was the primary basis for diagnosis of ear pathologies. It was discovered that the accuracy of the measurement of the null value obtained with an acoustic reflectometer depended on the line of sight from the instrument tip to the tympanic membrane. This problem was overcome by analyzing the shape of the acoustic reflectance curve, as shown in PCT Publication WO96/23293 and U.S. Pat. No. 5,699,809 (Combs et al.).
Another kind of acoustic reflectance measurement is shown in U.S. Pat. Nos. 5,594,174 and 5,651,351, to Douglas Keefe. These patents disclose a circuit that detects reflected waves separately from the incident waves. This separation permits the transfer function of the ear canal to be determined.
The accuracy of measurements of acoustic reflectance may be diminished due to noise. Noise may be caused, for example, by the response of an acoustic chamber to a change in frequency of the acoustic wave exciting the chamber or by noise in the received signal. In addition, normalization of the device by adjusting the received signals to compensate for the frequency response of the device limits the dynamic range which may be measured by the device. Finally, results may be unreliable due to user technique, particularly if the operator does not operate the device for a sufficient period of time to collect a sufficient amount of data for analysis. The present invention overcomes these difficulties to improve the sensitivity and reliable of the device.